1. Technical Field
The present invention concerns a circuit for achieving large ignition advance, limitation of the speed of revolutions in both directions of rotation, and for preventing backfiring in a magnetic ignition system comprising a flywheel and a three-legged iron core.
2. Description of the Related Art
For reasons of safety, the motor in a motor saw or similar device must not rotate in the wrong direction since this means that the saw chain also rotates in the wrong direction, leading to the risk of personal injury. If the fuel/air mixture in the motor is ignited significantly before the piston reaches its turning point when trying to start the motor, the motor can start to rotate backwards, since its kinetic energy is low during start, which means that the piston can be pressed downwards in the wrong direction of rotation. Such a process in a motor is known as xe2x80x9cbackfiringxe2x80x9d. In certain conditions, the motor can also reverse its direction of rotation during operation, which is known as xe2x80x9creverse direction runningxe2x80x9d. The risk for personal injury must be eliminated if the motor starts in the wrong direction.
In order to prevent backfiring, it is desired to ignite the mixture close to the upper turning point the piston (top dead centre) when running at low speed, since the kinetic energy of the motor is then low, this is known as xe2x80x9clow ignition advancexe2x80x9d. On the other hand, at high speed, it is desired to ignite the mixture earlier, since this extracts more power from the motor. This is known as using a xe2x80x9chigh ignition advancexe2x80x9d.
An ignition system with a large ignition advance (a large difference between the time of ignition at low speed and the time of ignition at high speed) thus makes it possible to reduce the risk of backfiring at low speed at the same time as making it possible to extract a great deal of power from the motor at high speed. However, this is difficult to achieve with one module that does not have a large ignition advance, since such a module is normally adjusted to an ignition position that is a compromise between starting ignition and ignition for high speed.
The maximum speed of revolutions of many hand-held tools is limited, something that is known as xe2x80x9cexcess speed protectionxe2x80x9d. This both increases personal safety and increases the lifetime of the motor. Individual solutions currently exist for achieving large ignition advance, a limitation of the speed of revolutions and for preventing backfiring in motors.
The present invention concerns a circuit for achieving large ignition advance, limitation of the speed in both directions of rotation, and for preventing backfiring and reverse direction running of a motor.
The circuit on which the invention is based concerns a magnetic ignition circuit that comprises a flywheel, a three-legged iron core, and a two-poled magnetic circuit. A triggering coil is arranged either on the first, second or both of these legs of the iron core in the direction of rotation of the flywheel, while a charging coil for generating a charging pulse for a charging circuit is arranged on the final leg of the iron core. The triggering coil generates a triggering pulse to a main switch before the charging pulse, when the rotation is in the forward direction.
In order to achieve a large ignition advance, and thus safety from backfiring, the triggering coil can be wound such that each turn covers the two first legs of the three-legged iron core in the direction of rotation. In this way the triggering pulse becomes broader. The disadvantage of this is that the flux passes through both legs, which affects the shape of the induced triggering pulse, something that can disturb the normal function. Another solution is to make the mechanical extent of the first leg greater, which gives a broader triggering pulse, which in turn makes possible a larger ignition advance. The disadvantage of this is that the system becomes larger and heavier.
The invention, which is intended to solve these problems, is characterised by a time-constant circuit, in this case an RC net, which is connected between the charging circuit and the main switch. Such a net is previously known and is often used as an excess speed protection. In the invention according to the present application the time-constant circuit controls a control switch that opens/blocks triggering pulses to the main switch, whereby the net can be used for limiting the speed of revolutions, in order to achieve safety from reverse direction running and to achieve large ignition advance in a cost-efficient manner without the ignition system becoming unnecessarily large and clumsy.
Other characteristics of the invention are specified in the accompanying claims.